The formula for the given joint variation is
... V = k·B·h
Solving for k gives
... k = V/(B·h)
Filling in the given values for the first pyramid, we find
... k = 56/(24·7) = 1/3
Using this to find B for the second pyramid, we see
... V = (1/3)B·h
... 81 m³ = (1/3)B·(9 m) = B·(3 m)
... (81 m³)/(3 m) = B = 27 m² . . . . corresponds to the 3rd choice